High intensity radiant gas burner

ABSTRACT

A radiant heat gas burner comprising a radiant element formed of a refractory ceramic honeycomb, and a porous gas injection block formed of refractory fibers can be operated at high turn-down ratios and at high temperatures. Still higher operating temperatures can be reached when the combustion chamber is either completely or partially filled by refractory fibers.

Sowards 1 1 HIGH INTENSITY RADIANT GAS BURNER [75] lnventor: DonaldMaurice Sowards,

Ashboume Hills, Claymont, Del.

[73] Assignee: E. I. duPont de Nemours and Company, Wilmington, Del.

22] Filed: Nov. 19, 1971 211 App]. No.: 200,498

[ 1 Aug. 7, 1973 3,087,041 4/1963 Vonk 431/328 3,199,573 8/1965 Flynn3,324,924 6/1967 Hailstone et al 431/328 Primary ExaminerCarroll B.Dority, 3r Att0rneyPaul R. Steyermark 57] ABSTRACT A radiant heat gasburner comprising a radiant element formed of a refractory ceramichoneycomb, and a porous gas injection block formed of refractory fiberscan 52 US. Cl. 431 28 is I} rm. Cl. F23d 145 /12 be high and high [58]Field of Search g 329 peratures. Still higher operating temperatures canbe reached when the combustion chamber is either com- [56] ReferencesCited pletely or partially filled by refractory fibers.

UNITED STATES PATENTS 14 Claims, 5 Drawing Figures 3,044,532; 7/1962Honger 431 329 5 r, (I, C /A I V/4 J l I 1 [I 4 j I 1 1A 3 V8 PATENIEUsum 101:2

FIG-l INVENTOR DONALD M. SOWARDS ATTORNEY PAIENTED M19 7 I975 SHEEI 2[if 2 FIG- FIG-

HIGH INTENSITY RADIANT GAS BURNER BACKGROUND OF THE INVENTION Thisinvention relates to high intensity radiant gas burners.

There are many types of radiant gas burners in use today, but most ofthem contain the following basic components: a gas inlet, a gasdistribution chamber, a

gas injection plate, a radiation element, and a combustion chamber,usually between the gas injection plate and the radiation element. Thedesigns of such burners and the materials used in their constructionvary to a greater or lesser extent, but the main objective always is toprovide the highest possible temperature of the radiant element withoutat the same time causing deformation, cracking, or other physical damageof the burners components.

US. Pat. No. 3,324,924 (to Hailstone et al.) discloses a radiant heatgas burner in which the radiant element is fabricated from ahoneycomb-shaped ceramic structure. This construction is said to permithigher temperatures than those attainable with metal grid elements and,further, provides a multiplicity of combustion zones, thereby resultingin a high combustion efficiency.

The radiant heat gas burner of US. Pat. No. 3,324,924 uses a gasinjection plate made of a uniformly perforated ceramic material, theinjection plate being in contact with the honeycomb radiant element. Theconventional hot plenum thus is eliminated.

Although prior art radiant heat burners, depending on their design andmaterials, could produce temperatures as high as about l,200-l,800 F.(about 649982 C.), higher temperatures of the radiant element often arerequired. There is a need, therefore, for a high intensity radiant heatgas burner which can be operated efficiently at high temperatures.

SUMMARY OF THE INVENTION According to this invention, there is provideda very efficient high intensity radiant gas burner, which can beoperated at temperatures as high as 3,200F. (about 1,760 C) and higher.The burner has the usual components, namely, a gas and air injectionmeans, a gas distribution chamber into which gas and air are delivered,an injection block, a hot plenum or combustion chamber, and a radiantelement. The radiant element is a refractory ceramichoneycomb; theinjection block is a porous material formed from refractory fibers; andthe combustion chamber may be either partially or completely filled withloose refractory fibers.

DESCRIPTION OF THE DRAWINGS FIG. 1 represents a vertical cross-sectionof one possible radiant heat burner of the present invention.

FIG. 2 represents a vertical cross-section of another radiant heatburner of the present invention. FIG. -3 represents a partial verticalcross-section of a radiant heat burner in which a ceramic honeycombseparates the gas injection plate from the combustion zone.

FIG. 4 is a partial vertical cross section of a radiant heat burner ofthe invention to which the cells of the radiant element are slantedrelative to the face of the element.

FIG. 5 is a partial vertical cross section of a radiant heat burner ofthe invention in which some of the cells are so slanted.

DETAILED DESCRIPTION OF THE INVENTION A radiant heat burner of thepresent invention can be built in many different configurations, shapes,and sizes, and using different materials of construction for theirnoncritical components. The invention will be better understood byreference to the drawings, all showing the preferred embodiment, wherethe combustion chamber is either completely or partially filled withrefractory fibers.

In FIG. 1, the burner is shown as comprising a housing 6, an inlet 7 forintroducing a mixture of gas and air to the gas distribution chamber 1,a porous gas injection block 2 formed from refractory ceramic fibers, tobe described later, a combustion chamber 8, a loose layer or a batt ofrefractory ceramic fibers '3 within the combustion chamber, and aradiant element 4 formed of a refractory honeycomb; One surface of theporous gas injection block 2 forms a portion of the defining wall of thegas distribution chamber. The other surface forms a portion of thedefining wall of the combustion chamber.

The term combustion chamber is used throughout this specification ratherthan the term hot plenum. It is, of course, well understood that theactual combustion of the fuel also takes place within the cells of thehoneycomb.

The thickness of the gas injection block 2 will vary .with each specificburner design and will depend to some extent on the material density,which can vary from about 5 to 25 lb./cu.ft. Theporosity of the materialdetermines its permeability to gas flow. The gas flow is more uniform atlower densities. The thermal insulating properties of the block 2 alsoare better at lower densities, but the durability of the material isbetter at higher densities. The optimum density will be chosen on thebasis of the above considerations. The thickness of the injection block2 must be sufficient for adequate thermal insulation of the combustionchamber from the gas distribution chamber. Otherwise, gas could igniteprematurely in the gas distribution cham ber.

The combustion chamber 8 will have a height of about one-sixteenth to 1inch, the optimum height being about three-eighths inch. The innersurface of the radiant element 4 forms a portion of the defining wall ofthe combustion chamber.

The layer of refractory fibers 3 can either fill the combustion zonepartially, for example, as shown in FIGS. 1 and 2, or completely. Thesefibers, which are assembled as a dry, loose fiber tow, have a bulkpacking density of less than I lb./cu.ft. to 3 lb./cu.ft. One of thepurposes of these fibers is to increase the tum-down ratio of theburner. The turn-down ratio can be defined as the ratio of the amountsof fuel which can be fed to the burner at the high and the low stableoperating conditions. The tum-down ratio of a conventional radiant heatburner can be within the range of 4-8. The high intensity burner of thepresent invention, which contains the refractory fibers 3 in thecombustion zone, can reach a tum-down ratio as high as 12-15. In theextreme case, no refractory fibers are placed in the combustion chamber,the only refractory fibers in the chamber being those on the surface ofthe injection block forming a portion of the defining wall of thecombustion chamber. This case is illustrated in Example 4, below.

The housing 6 canobe built of any convenient material, including castiron and clay; and the size of the gas delivery chamber 1 is notcritical. The side walls of the combustion chamber, between the gasdistribution block and the refractory honeycomb, are made of arefractory material able to withstand the burners temperature. The sidewalls need not be continuous and can be small spacer piers.

FIG. 3 shows a detail of another possible configuration of a radiant gasburner of the present invention, wherein a refractory honeycomb 9 isplaced between the gas injection block 2 and the combustion chamber 8.This refractory honeycomb directs the incoming gas at an angle normal tothe surface of the injection block and thus decreases gas scatteringwithin the combustion chamber, with corresponding reduction of internalradiation.

FIGS. 4 and 5 illustrate two other embodiments of the invention in whichall or some of the cells of the radiant elements are slanted relative tothe honeycomb open surface of the element.

' Methods for making the honeycomb structures used as radiant elementsin the burner of this invention are known in the art. One suitablemethod is disclosed in British Patent 931 ,096. This method comprisesforming a plasticized raw material mix containing finely dividedsinterable particles of a refractory material, plasticizing ingredientsand volatile viscosity adjustment media into a thin film or sheetmaterial. The sheet material is then corrugated and honeycomb structuresare fabricated by placing sheets together so that the nodes of one sheetare in'contact with nodes of another corrugated sheet or with anon-corrugated sheet. The structure is then fired to sinteringtemperatures. Examples of sinterable materials which can be used arealumina, zirconia, cordierite, zircon, barium titanate and magnesia.

Another suitable method for making the honeycomb structures is disclosedin U.S. Pat. No. 3,112,184. In this method a suspension containingpulverized ceramic material and a binder is coated on each side of aflexible carrier. The carrier is corrugated and the corrugated materialis used to fabricate honeycomb structures. The green structure is thenfired to sinter the ceramic particles. As described in the patent, thepurpose of the carrier is to provide support for the unfired coating toallow it to be formed to the desired shape prior to the firing step. Thecarrier can be either an inorganic or organic material although thelatter is preferred since it burns out on firing and does not appear inthe final product. Also, preferred for use as carriers according to thismethod are fibrous materials containing a multitude of holes which passthrough the carrier from one surface to the opposite surface and whichcan be completely filled by the ceramic slurry to produce an unlaminatedwall upon firing.

A particularly suitable method for making the honeycomb structures isthat disclosed in Belgian Patent 612,535, issued July 11, 1962. In thismethod aluminum foil is fabricated into a honeycomb structure of thedesired shape and is fired under controlled conditions to oxidize thealuminum to alpha alumina. Prior to the firing step the aluminum foil iscoated with an agent, identified in the patent as a fluxing agent,-whichserves to prevent inhibition of oxidation due to oxide scum formation onthe surface of the aluminum. Examples of fluxing agents disclosed in thepatent as being suitable include alkali metal and alkaline earth metaloxides and precursors of these oxides, i.e.,' compounds which yield theoxides on firing. A particularly suitable agent is sodium oxide which isapplied as sodium silicate. 1

The honeycomb products resulting from this process are substantiallypure alpha alumina. If desired, the chemical composition of thestructures can be modified by including in the coating compositionfinely divided particles of filler refractory oxide. The fillerrefractories may, if desired, be one or more of those which will reactwith the alumina as it is formed. If a reactive filler such as magnesiaand/or silica is used, the honeycomb structure will contain thecorresponding reaction product such as spinel, cordierite or mullite.The products of this process are characterized by outstanding strengthand thermal shock resistance.

As disclosed in the Belgian patent, the honeycomb structures may befabricated by corrugating sheets of aluminum foil coated with fluxingagent and placing the coated sheets together node to node. Where sodiumsilicate solution is used as the fluxing agent, the body will havesufficient greenstrength to maintain its shape until it is fired.Alternatively, the honeycomb structure may first be fabricated from thealuminum foil using methods well known in the art and described in thepatent literature. Suitable prefabricated aluminum honeycomb structuresfor use in this process are available commercially and may be purchasedfrom Hexcel Corporation or Bloomingdale Rubber Division of AmericanCyanamid, both of Havre de Grace, Md.

An improvement in the process for making honeycomb structures by themethod of the Belgian patent is disclosed in US. Pat. No. 3,473,938 (toOberlin). In the process of this patent the composition used to coat thealuminum honeycomb structure contains, in addition to the fluxing agentand filler refractory, if any, small amounts of a vanadium compound. Theproducts of the Belgian patent are characterized by having adouble-walled structure. The double-wall results from the fact that thealuminum foil, as it melts, flows outwardly through the oxide formed onthe outer surfaces of the foil and is oxidized at the outer surface ofthe oxide layer, thus leaving a large void in the final productcorresponding approximately in thickness to the thickness of theoriginal aluminum foil. The inclusion of the vanadium compound in thecoating composition causes the formation of bridges of refractorymaterial between these double walls, resulting in a product having evengreater strength and thermal shock resistance than the products of theBelgian patent.

The structural design parameters for the honeycomb radiant elements arethe diameter of the cells and the thickness of the walls of the cells.

The diameter of the cells 5 of the honeycomb can vary from aboutone-sixteenth to one-fourth inch. A cell diameter of about one-eighthinch has been found to be most practical for the radiant elements usedin the burners of the present invention. Structures with smaller celldiameters than one-sixteenth inch can be used but are more difficult tofabricate. Structures having cells larger than about one-fourth inch areordinarily not desirable, simply because they become too bulky forconvenient use. As will be discussed below, it is desirable that thehoneycombelement have a cell length-to-diameter ratio in the range ofabout 8:1 in order to efficiently collimate the radiation and to provideradiant emissivity conditions approaching those of a black body. Thus,where the cell diameters are greater than about one-fourth inch, thethicknesses of the structure required to provide the desiredlength-todiameter ratio will be so great as to make the structure toobulky for ease of handling and installation. Furthermore, as the celldiameter increases, the unit becomes more susceptible to flame blowoutby cold drafts.

The range of cell diameters given above are the nominal sizes, i.e.,ignoring the wall thickness. It is perhaps more accurate to say that, asa practical matter, the number of cells per inch will range from a lowerlimit of about 2% cells per inch to a maximum of about 15 cells perinch. Wall thicknesses will vary from a minimum of about 0.005 in.,where a honeycomb structure having 15 cells per inch is used, to amaximum of about 0.10 in., where a honeycomb structure having 2% cellsper inch is used. Of course the wall thickness in a honeycomb structurehaving 15 cells per inch can be greater than the minimum stated but itshould be less than that necessary to provide a structure having an openarea normal to the cell axes of at least about 40 percent. Similarly,the wall thickness in a structure having 2% cells per inch can be lessthan the maximum but the thickness must be great enough to provide astructure with a maximum open area normal to the cellaxes of about 95percent.

- As indicated above, the ratio of cell length to diameter in thehoneycomb-shaped radiant elements is an important design factor. it iswell known that the effective emissivity of a cavity such as a honeycombcell approaches unity as the ratio of the length to the diameter of thecavity increases. In other words, the radiation characteristics ofhoneycomb cells approach those of a black" body as the ratio of lengthto diameter increases. It was shown, for instance, in U.S. Pat. No.3,324,924 that at identical b.t.u. input and other operating conditions,the output of radiant heat energy translated into electrical energyalmost doubled on increasing the length-to-diameter ratio from 2:1 to10:1.

It is to be understood that, while FIGS. 1, 2 and 3 show all the ceramichoneycomb cells parallel to the gas stream and normal to the honeycombopen surfaces, the cells can also be either completely slanted as shownat 5B and 5A in FIG. 5 or slanted in part and normal in part as shown at5B and 5A in FIG. 5. The choice of the proper refractory honeycomb forthe radiant element can be easily made by an engineer familiarwithradiant gas burners.

The refractory fibers which can be used to form the injection block 2and the loose fiber batt 3 can be made of any. refractory material,capable of withstanding temperatures in the neighborhood of l,760 C. andabove. Usually, such fibers will be made of alumina, but certain highlyrefractory silica or zirconia fibers also could be used. Several typesof alumina fibers either are presently. on the market and can be readilyobtained from commercial sources, or can be prepared according topublished processes. The manufacturers of alumina fibers include, amongothers, Tyco Corporation, Union'Carbide Corporation, and Babcock &Wilcox Co. Alumina fibers have been disclosed in U.S. Pat. Nos.3,082,099, 3,385,915, and 3,180,741. Zirconia fibers can be obtainedfrom Union Carbide Corporation.

The injection block 2 is conveniently formed by filtering at a reducedpressure a slurry of the fibers in water and drying the resulting filtercake either at ambient temperature or at an elevated temperature. Thethickness of the slurry is so adjusted that a filter cake of desiredporosity is obtained. Usually the concentration of the fibers in theslurry will vary from about 1 to 5 weight percent. Although the dryfilter cake has sufficient mechanical strength to be cut and shaped asrequired, it is more practical to choose the equipment so that a dryfilter cake of the proper size and shape will be formed. The porosity ofthe injector block material should be such as to allow a pressure dropof the gas and air mixture on passing into the combustion zone of about2-20 psig.

When a radiant gas burner of this invention is als provided with arefractory honeycomb insert 9, adjacent to the injection block 2, asshown in FIG. 3, the construction and cell diameters of this insert arethe same as those described above for the radiant elements. However, thecell length-to-diameter ratio is much less critical in this case. Thethicknes of the honeycomb insert 9 can vary within rather broad limitsbut usually will be from one-eighth to one-half inch. In this case, thecells will be normal to the open surfaces of the honeycomb.

lnithe operation of a radiant gas burner of the present invention, afuel and air mixture is introduced into the gas distribution chamber 1through the port 7. While separate gas and air inlets can be usedsatisfactorily, an injection nozzle is usually preferred. The fuel is ahydrocarbon gas, such as natural gas, a C,--C saturated hydrocarbon, ora mixture of C,C saturated hydrocarbons. Such hydrocarbons includemethane, ethane, propane, and butane. The burner of the presentinvention can also be provided with an evaporator in the gasdistribution chamber. When an evaporator is provided, the burner canalso be used with certain liquid fuels such as light naphthas.

The air and fuel components of the mixture are usually injected in theirstoichiometric ratio, minor deviations, of the order of plus or minus 10percent, being acceptable. With a larger excess of the fuel, theresulting fuel waste would be economically undesirable, and anenvironment pollution hazard would be created by the presence of eitherunburned or incompletely burned fuel in the exhaust gases. A largerexcess of air also is undesirable because it may cause cooling of thecombustion chamber below the optimum temperature range, and because alarger volume of air would require larger and more expensive pumps,which would thus operate at only fractional efficiency.

The burner of this invention is stable in operation over a wide range offeed rates of the gas/air mixture. The efficiency of the burner is ofthe order of 35 to v depending primarily on the desired operatingtemperature. Efficiency is defined asthe ratio of radiant heat leavingthe burner to the calorific value of the gas entering the burner. Theburner of this invention is useful for space heating and for domestic orindustrial heating applications. The refractory radiant element can bemodified by the presence of suitable substances to alter the wavelengthsof the radiation from those normally emitted by the refractory at giventemperatures. Thus, the radiant element can be designed so as to emitvisible light. Compounds of zirconium, cerium, thorium, manganese,copper, cobalt, calcium, barium, stron-' tium, lithium, sodium,potassium, and the like can be used for this purpose. These substancescan be coated on the fired radiant element or can be included ascomponents of the unfired structure.

This invention is now illustrated by representative examples of certainpreferred embodiments thereof.

EXAMPLE l An injector block, 2 inches by 2 inches and threefourths inchthick, was prepared as a filter cake by vacuum filtration of asuspension of alpha alumina fibers in water. The fibers were about 0.008inch in diameter by one-fourth to 1 inch in length. The slurry contained4.8 percent fibers by weight and the thickness of the cake was attainedby adding the proper amount of slurry during the filtration. The drieddensity of the injector block was 11.3 pounds per cubic foot.

The injector block was attached with refractory cement as the cover of acast iron box, equipped with gas and air inlet tubes, to form a coldplenum 2% inches deep. The refractory cement casing was extended up thesides and three-eighths inch above the top of the injector block to formthe walls of the combustion chamber.

A inch thick disc of alumina fibers, about 1 to 2 inches long, wasformed in the bottom of the combustion chamber by loosely sprinklingfibers, often moistening with aluminum chlorohydrate, into the hotplenum cavity. The aluminum salt serves to bind the loosely packedfibers together. The packing density of this disc is about 0.6 poundsper cubic foot.

A radiant element was provided by laying a 2% inch by 2% inch piece ofalpha alumina ceramic honeycomb, one inch thick with inch diametercells, upon the edges of the extended refractory cement rim.

Using natural gas, with a rating of about 1,040 BTU/- cu.ft., mixed with10 volumes of air, this burner began to show stable performance at atotal gas flow rate of about cu.ft./hr. and continued to operate well upto a flow above 200 cu.ft./hr. The hot plenum temperature as indicatedwith an optical pyrometer ranged from 1,200 to 1,760 C. with increasinggas flow.

A 12-inch diameter copper water calorimeter, with a carbon black frontcoating, indicated a radiant heating efficiency of near 70 percent atlower temperatures, decreasing to 38 percent at the highest temperature.This is measured as the BTU content of the natural gas feed relative tothe BTU required to raise the water temperature in the calorimeter tothe measured level.

EXAMPLE 2 A burner was constructed as in Example 1, except thecombustion chamber walls were formed with high temperature refractoryblocks and the combustion chamber was one-half inch thick. Zirconiafibers (Union Carbide Corp.) and alumina fibers containing 6 to 8 weightpercent of chromium, manganese, or cobalt, added as the respectiveoxides, were also tested as fillers for the combustion chamber. About 1gram of loose fiber was placed in the hot plenum cavity and covered withthe ceramic honeycomb. The fibers thus filled the combustion chamber.With a gas flow of 115 cu.ft./hr. the following results were obtainedfor several plenum packings: I

Combustion Combustion Chamber Chamber Heatin Additive Temperature,C.Efliciency,

Alumina, only 1450 67 Alumina with 1650 56 chromium Alumina with 1750 44manganese Alumina with 1600 57 cobalt Zirconia 1550 60 EXAMPLE 3 Aburner similar to that of Example 1 was con- EXAMPLE 4 A 1% inch thickinjector block was prepared using zirconia fibers. It had a density of22 lbs./cu.ft. This was used in a burner similar to that of Example 1,except no loose fiber was placed in the combustion chamber.

With a total gas flow rate of 140 cu.ft./hr. the combustion chambertemperature was 1,420 C. and the heating efficiency was 64 percent.

EXAMPLE 5 The burner of Example 2 with alumina fibers in the combustionchamber was used, except that 1% inches thick honeycomb with 3/16 inch'cells and 2 inches thick honeycomb with V4 inch cells were used as theradiant element.

At a total gas flow rate of 125 cu.ft./hr. the following results wereobtained:

Radiant Combustion Heating Element Chamber Temp. Efficiency 3/16" celll5l0C.

'A" cell 1485C. 36%

I claim:

1. A radiant heat gas burner comprising:

a. a housing;

b. at one end of the housing, a radiant element formed from a ceramic,refractory, open-celled honeycomb;

c. a combustion chamber, a portion of the defining wall of said chamberbeing formed by one surface of said radiant element;

(1. a gas injection system comprising a porous block formed fromrefractory ceramic fibers having a density of about 5 to 25 lb./cu.ft.,one surface of said system forming a portion of the defining wall of thecombustion chamber;

e. at the other end of the housing, a gas distribution chamber, aportion of the defining wall of said chamber being formed by anothersurface of the gas injection system, said chamber being fitted with ameans for the introduction of gas and air.

2. The radiant heat gas burner of claim 1 wherein the gas injectionsystem is constituted of the porous block formed from refractory fibers.

3. The radiant heat gas burner of claim 1 wherein the number of cells inthe radiant heat element is comprised within the range of 2% to 15 perinch; the open cell area of said element normal to the cell axes iswithin the range of about 40 topercent; and the length-to-diameter ratioof the cells is at least 2:1.

4. The radiant heat burner of claim 1 wherein the cells of the elementare slanted to the honeycomb open surfaces.

5. The radiant heat burner of claim 1 wherein the cells of the radiantelement are slanted in part and normal in part to the honeycomb opensurfaces.

6. The radiant heat gas burner of claim 1, wherein the combustionchamber is filled to at least part of its height with a loose tow ofrefractory fibers, the packing density of the fibers being less than 1lb./cu.ft. to 3 lb./cu.ft. of bulk material.

7. The radiant heat gas burner of claim 6 wherein the gas injectionsystem is constituted of a planar refractory ceramic honeycomb structurein contact with the porous block formed from refractory fibers, soarranged that the surface of said porous block opposite to the surfacedefining a portion of the combustion zone is in contact with one end ofeach cell of the honeycomb structure, the cells of the honeycomb beingnormal to its open surfaces.

8. The radiant heat burner of claim 6 wherein the refractory fiber towcompletely fills the combustion 12. The radiant heat burner of claim 10wherein the porous injection block is made of alumina fibers.

13. The radiant heat burner of claim 1 wherein the porosity of theporous gas injection block is such that the pressure drop of the gas andair mixture on passing from the gas distribution chamber into thecombustion chamber is about 2-20 psig.

14. The radiant heat burner of claim 13 wherein the thickness of theceramic honeycomb in the gas injection system is about one-eighth toone-half inch.

2. The radiant heat gas burner of claim 1 wherein the gas injectionsystem is constituted of the porous block formed from refractory fibers.3. The radiant heat gas burner of claim 1 wherein the number of cells inthe radiant heat element is comprised within the range of 2 2/3 to 15per inch; the open cell area of said element normal to the cell axes iswithin the range of about 40 to 95 percent; and the length-to-diameterratio of the cells is at least 2:1.
 4. The radiant heat burner of claim1 wherein the cells of the element are slanted to the honeycomb opensurfaces.
 5. The radiant heat burner of claim 1 wherein the cells of theradiant element are slanted in part and normal in part to the honeycombopen surfaces.
 6. The radiant heat gas burner of claim 1, wherein thecombustion chamber is filled to at least part of its height with a loosetow of refractory fibers, the packing density of the fibers being lessthan 1 lb./cu.ft. to 3 lb./cu.ft. of bulk material.
 7. The radiant heatgas burner of claim 6 wherein the gas injection system is constituted ofa planar refractory ceramic honeycomb structure in contact with theporous block formed from refractory fibers, so arranged that the surfaceof said porous block opposite to the surface defining a portion of thecombustion zone is in contact with one end of each cell of the honeycombstructure, the cells of the honeycomb being normal to its open surfaces.8. The radiant heat burner of claim 6 wherein the refractory fiber towcompletely fills the combustion chamber.
 9. The radiant heat burner ofclaim 6 wherein the refractory fiber tow in the combustion chamber isalumina fibers.
 10. The radiant heat gas burner of claim 1 wherein theheight of the combustion zone is one-sixteenth to 1 inch, and the celldiameter of the radiant element is one-sixteenth to one-fourth inch. 11.The radiant heat gas burner of claim 10 wherein the celllength-to-diameter ratio of the radiant element is about 8:1.
 12. Theradiant heat burner of claim 10 wherein the porous injection block ismade of alumina fibers.
 13. The radiant heat burner of claim 1 whereinthe porosity of the porous gas injection block is such that the pressuredrop of the gas and air mixture on passing from the gas distributionchamber into the combustion chamber is about 2-20 psig.
 14. The radiantheat burner of claim 13 wherein the thickness of the ceramic honeycombin the gas injection system is about one-eighth to one-half inch.